Hi Allsorts. You don't say whether these groups are specific interest groups or simply separate groups of friends. I wonder if people have invited their friends as they see the group as a friendly bunch and that their friends would benefit from being part of it? Or whether they share the same interest. Have you got to know the new 'members' as i am sure they stick to their friends initially, but, as they have joined are willing to get to know others.
With regard to holidays, i have had a holiday with a group of friends and was surprised as to who i actually got along with better, oddly, not my closest friend in the group. We are all different people, some early birds, some night owls, some like a long lazy breakfast, others just a coffee and go and so on, therefore i found myself spending time with the person i knew the least but had similar wants from the time away and we often broke away from the main group to pursue these interests.
There is no reason that you can't ask a couple of your closest friends to meet up for a coffee and be honest that you really can't catch up properly in such a big group.